According to the conventional technology, maintaining an aerobic condition and settling solid matters in water have not been accomplished at the same time. More particularly, when it is desired to supply oxygen into water, there has been no alternative other than letting air pass through the water in a bubbling manner on a large scale. On the other hand, the settlement of solid matters is done by keeping the water stationary or by causing a centrifugal motion of the water. These two phenomena can not occur at the same time. Considering the sewage treatment currently carried out as a concrete example, treated water through an aeration tank contains a large number of microorganisms and small metazoans and is in great demand of oxygen. Accordingly, in a precipitation or settling tank, it is inevitable that oxygen in the treated water is used up in a short period of time and the treated water comes to a state of containing no oxygen. As concerns supplying oxygen while removing carbonic acid gas, according to the conventional technology, any measures except ventilation with strong stirring of water have been considered to be impossible.
The present invention is an apparatus which can perform settlement in an aerobic manner.
Letting air effectively pass through water has been considered to be best for bringing the water to an aerobic condition, and to bring about such a condition involves stirring of the water. On the other hand, when causing settlement, water has to be kept stationary and, therefore, it is impossible to let air pass therethrough. However, the applicant of the present invention has filed another international application entitled "MULTIPLE-GAS-PHASE BREEDING METHOD" (PCT/JP88/00925). In that application, a new method of supplying oxygen into water, or a ventilation method which is sure and involves less stirring effect, has been proposed. The method proposes holding air in plural piles under water to physically increase a contact surface area between the air and water. This method has the characteristic feature that natural water surfaces are artificially provided under water in countless numbers and, at the same time, breeding places for life are countlessly provided.